Films of pure Cu and of Cu alloy containing 2.5 at. % of Zr [abbreviated as Cu(2.5 at. % Zr)] were deposited on SiO2/Si substrates by magnetron sputtering. Samples were subsequently annealed at temperatures ranging from 500 to 800 °C in vacuum (2×10-5 Torr) for 30 min and analyzed by Rutherford backscattering spectrometry, x-ray photoelectron spectroscopy, and glancing incident angle x-ray diffraction. Resistivity of both pure Cu and Cu alloy films, before and after annealing, was measured at room temperature by using a standard four-point probe technique. Upon annealing, the added Zr atoms in Cu(2.5 at. % Zr) diffuse to both the free surface and the alloy/SiO2 interface and react with the residual oxygen in the vacuum system and with the SiO2 to form a ZrO2 layer. At the interface, a self-grown ZrO2 layer forms upon annealing at 700 °C that hinders Cu from diffusion into the SiO2, while Cu diffusion into SiO2 is apparent for pure Cu on SiO2 at this temperature. The room-temperature resistivity of the as-deposited Cu(2.5 at. % Zr) film is 21.8 μΩ cm and decreases to about 6.2 μΩ cm upon vacuum annealing at 700 °C for 30 min. This value is still thrice that of the pure Cu film similarly treated. The relation between the diffusion of the added Zr and the characteristics of Cu(2.5 at. % Zr)/SiO2 interface, self-formed surface layer, and the resistivity change, is discussed.